The study used various geochemical techniques to estimate fluid-rock mass transfer rates in reservoir rocks consisting primarily of Bandelier Tuff. Equilibrium constraints, fluid reservoir volume and discharge rates suggest that fluid residence time is approximately 2 _ 103 years, or <0.2% of the total age of the hydrothermal system. These results are suggestive of a geochemically and isotopically open system, in which Cl was the only aqueous component not controlled by mineral equilibrium.